The invention relates to a method for monitoring, documenting and/or controlling an injection molding machine having an injection mold into which a melt is introduced, the viscosity of the melt in the injection mold being ascertained directly by way of the respective quotients of shear stress and shear rate on the basis of pressure differences, the geometry of the cavity and the flow rate of the melt.
To keep a check on the consistency of plastic material, pasty compositions, emulsions and liquids, the viscosity is determined in dependence on the shear rate. The viscosity describes dynamic shear stresses caused by internal friction in moving liquids or in pasty compositions. The definition of viscosity is based on Newton's theory, which states that the shear stress is proportional to the shear rate. The proportionality factor is in this case referred to as viscosity (shear viscosity). The two terms shear stress and shear rate can be explained by the example of a liquid film of a thickness d which rests on one bounding surface and is moved on the other at a velocity v on account of the shear force acting on it. The shear stress corresponds to the shear force per unit area and the shear rate corresponds to the change in the velocity of displacement of one bounding surface in relation to the other divided by the distance between the two bounding surfaces.
In order to determine a relationship between the shear rate and the shear stress or the viscosity, viscosity measurements must be carried out for various shear rates. The viscosity can be determined by the Hagen/Poiseuille method by means of a capillary through which the liquid or composition to be investigated flows on account of a charging pressure. The shear stress and the shear rate, and consequently the viscosity, can be determined from the rate of flow, the charging pressure, the change in pressure along the capillary and the cross section of the capillary. Because the shear rate depends both on the charging pressure and on the cross section of the capillary, measurements for various shear rates can be carried out by changing these variables.
U.S. registered design 3,438,158 discloses, for example, determining the flow stress and the viscosity of a non-Newtonian fluid. This involves pumping the fluid through a pipe of known diameter at a known flow rate. By repeated measurement of the pressure differences or the pressure drop along a given length of pipe under different conditions in each case, the aforementioned rheological parameters can be determined.
DE 10 2005 032 367 A1 already discloses a method for monitoring and/or controlling the filling with melt of at least one cavity of an injection molding machine. According to this document, material or viscosity fluctuations can be indirectly ascertained, monitored and controlled, to be precise by analyzing the differences in the time for filling with the melt from cycle to cycle. Although such a method allows differences in viscosity to be detected, and possibly corrected, it cannot be used to ascertain a genuine viscosity profile in the physical unit of pascals per second (Pa s). But to be able to quantify a change in viscosity, it must be known in the genuine physical unit.
A further method for monitoring injection molding processes is shown by U.S. Pat. No. 4,833,910. For this purpose, two pressure sensors are positioned in the cavity at a known distance in the direction of flow of the melt. The viscosity is determined by way of the pressure difference ascertained in this way, the time difference, the radius of the channel and the distance between the two sensors.
In classical rheometry, the viscosity is ascertained as a quotient of shear stress and shear rate, usually for material determination in the laboratory. Serving for this purpose are so-called rheometers, which have a die and an exactly defined melt channel for ascertaining the viscosity, with a contrast here in comparison with an injection molding process in that isothermal conditions prevail. That is to say that both the metal die and the plastics melt are at the same temperature.
Serving for measurement in a rheometer are two melt pressure sensors (no mold internal pressure sensors), which are arranged a certain distance apart and measure the pressure drop over this distance. The shear stress can then be calculated on the basis of the geometry of the melt channel, which may be designed for example as a bore or as a rectangular channel, and on the basis of the pressure drop. In this case, the melt discharge through the die is forced out (extruded) at different rates or under different pressures, with the result that different pressure gradients (Δp) are obtained. Each individual pressure gradient produces a shear stress of its own, and consequently a value in the profile of a viscosity curve.
At the same time, the corresponding shear rate is calculated once again on the basis of the geometry of the melt channel and on the basis of the time that elapses while the melt passes from the first melt pressure sensor to the second melt pressure sensor.
Finally, there is also a more simple method than that for determining viscosities, one in which only one melt pressure sensor is used. It measures the pressure drop from the sensor to atmospheric pressure of 1 bar. In the case of this method, however, a computational correction must be made—the so-called “Bagley correction”—in order to compensate for run-out pressure losses. These run-out pressure losses occur when the melt leaves the channel into the open and expands. Otherwise, the further procedure corresponds to the method of a rheometer.
The object of the present invention is to monitor, and possibly control, an injection molding process under practical conditions.